Active vehicle suspension unit

ABSTRACT

A hydraulic shock absorber with a generally vertical cylinder encasing therein hydraulic liquid and a piston which partitions the interior of the cylinder into two liquid chambers, an upper and lower chamber. Improved damping control is achieved by a circuit forming a connection between the two liquid chambers, a further circuit connecting the reservoir to the lower liquid chamber, each of the two circuits including a controllable first check valve and second check valve, respectively, that can take two states of operation under the action of a control device. The first state resulting in locking the valve in a closed position, cutting off the forward flow through the check valve, and in a second state allowing a free forward flow through the check valve, whereas the check valve in the first state as well as in the second state blocks the reverse flow of both circuits.

BACKGROUND OF THE INVENTION

The present invention relates to vehicle suspension systems and moreparticularly, to a vehicle suspension system in which control meansincorporating a micro processor or computer controls the damping of ashock absorber to optimize riding and handling characteristics under awide range of driving conditions. The control means comprise sensorswhich provide signals to the micro processor which are representative ofthe vehicle riding comfort. The control means actively affect the shockabsorber operation utilizing programmed algorithms.

STATE OF THE ART

In the U.S. Pat. No. 4,154,461, May 15, 1979, a concept of a dual stateshock absorber for a vehicle wheel is introduced.

In said patent there is arranged a throttling valve, combining the upperand lower fluid chambers of a shock absorber cylinder, said valve beingfixed in one of two different positions under the influence of anexternal control means. By this arrangement the shock absorber wouldoffer either "soft", i.e. low damping or "stiff", i.e. high damping,respectively, to resist either compressive or expansive movements of theshock absorber.

The decision whether the damping should be stiff or soft is madedependent on the sensor signals y, representing the vehicle verticalvelocity (positive upwards), and the signal indicating the compressionor expansion sense of the shock absorber as expressed by (y-x). Here xis the vertical displacement upwards of the wheel and therefore (y-x) ispositive for expansion and negative for compression of the shockabsorber.

It is also shown by said patent that the decision about soft or stiffdamping could be made entirely dependent on the sign of the variableS=y·(y-x). Obviously the arrangement according to said patent would callfor two sensors; one sensor being an accelerometer on the car body toyield y and one sensor indicating the compression or expansion of theshock absorber.

The present invention relates to a hydraulic shock absorber including agenerally vertical cylinder receiving therein hydraulic liquid, a pistonworking in the cylinder and partioning the interior thereof into twoliquid chambers,

an upper and a lower chamber,

two sets of one or more damping force generating check valves mounted onthe piston, one set generating the damping force in the expansion strokeand another set allowing free flow from the lower to the upper chamberin the compression stroke,

a piston rod secured to the piston extending through the end of thecylnder to the outside, said sets of valves constituting connectionsbetween the two liquid chambers, a liquid reservoir connected to thelower liquid chamber by a third set of check valves generating thedamping force in the compression stroke and allowing a return free flowfrom the reservoir to the lower chamber in the expansion stroke.

The invention presents a mechanical design to improve the speed andaccuracy of the damping control and to simplify the sensor installation.This is achieved by a circuit forming a connection between the twoliquid chambers, a further circuit connecting the reservoir to the lowerliquid chamber, each of the two circuits including a controllable firstcheck valve and second check valve, respectively, that can each take twostates of operation under the action of a control means, in a firststate locking the valve in a closed position cutting off the forwardflow through the check valve, and in a second state allowing a freeforward flow through the check valve, whereas the check valve in thefirst state as well as in the second state block the reverse flow ofboth circuits.

In a second embodiment the control means comprise a locking devicekeeping the check valve head in sealing engagement with the valve seatto cut off the forward flow.

In a third embodiment the locking device comprises a rocking leverbarring the valve head in its sealed engagement in one position andsetting the valve head free in one or more further positions.

In a fourth embodiment the control means for the check valves compriseelectric relays.

In a fifth embodiment the control means are under the action of signalsfrom sensors for stating the vertical speed and/or acceleration of thevehicle body or the wheel, said signals being entered into a CPU microprocessor comprised in the control means.

In a sixty embodiment the control means is programmed to put the checkvalves in a basic mode or zero mode, Mode 0, when the accelerationsignal from the vehicle body or the wheel is within predeterminedvalues, to impart a free state, the second state, allowing free forwardflow to both check valves in case the vehicle body acceleration signalis not exceeding a predetermined limit value of the acceleration signal,and to impart a locked state, the first state, to both check valves incase the wheel acceleration signal is not exceeding a predeterminedlimit value of the acceleration signal.

In a seventh embodiment the control means is programmed to put the checkvalves in a first mode, Mode 1, when any of the acceleration signalsexceeds the high limit value, to impart a free, second state to thefirst check valve and a locked, first state to the second check valve,meaning a soft compression and a stiff expansion of the shock absorber.

In an eighth embodiment the control means are programmed to put thecheck valves in a second mode, Mode 2, when any of the accelerationsignals passes below the low limit value, to impart a locked, firststate to the first check valve and a free, second state to the secondcheck valve, meaning stiff compression and a soft expansion of the shockabsorber.

In a nineth embodiment the control means are programmed to retain Mode 1or Mode 2 for a predetermined time after the acceleration value hasreentered within the acceleration tolerance band, defined by the limitvalues, and after the expiration of the predetermined time to return toMode 0.

In a tenth embodiment the control means are programmed to put the checkvalves in a third mode, Mode 3, when a signal representing the countingof the number of cycles within a given time interval exceeds apredetermined value, or the acceleration signal time derivative exceedsa given value, said Mode 3 imparting to both check valves a locked,first state.

The invention further teaches principles of electronic control means tobalance the conflicting requirements or ride comfort and road holding aswell as bringing the suspension control in harmony with braking andsteering actions.

DRAWINGS

The invention will now be described in more detail referring to thedrawings in which

FIG. 1 is a schematic drawing of a shock absorber according to theinvention,

FIG. 2 is a sketch of an upward edge as hit by the wheel, and

FIG. 3 is a corresponding downward edge.

THE MECHANICAL DESIGN

Reference is made to FIG. 1. The patent may be applied to differentdesigns of conventional shock absorbers. FIG. 1 refers, however,schematically to an absorber of common design where a piston 6 moves ina cylinder 7. The piston divides the space in the cylinder so as to havean upper chamber 1 and a lower chamber 2.

During compression the piston rod 8 is introduced further into thecylinder 7, whereby the cylinder cannot contain as much fluid as before.The excess fluid in connection with a compressive movement istransferred to the reservoir 3.

In the compressive movement the main flow resistance is offered by oneor more check valves 5. Valves 4 in the piston 6 may or may not offer animportant flow resistance to the flux from the lower chamber 2 to theupper chamber 1.

At an expansive movement the valves 4 are important for the damping,whereas opposite valves 5 must not restrict the return flow from thereservoir 3 to chamber 2, which would cause a harmful partial vacuum inchamber 2.

The valves 4 and 5 correspond entirely to the throttling of the flow inany conventional shock absorber. They may or may not be springloaded soas to vary their flow cross section as a function of the pressuredifference across the valves.

The present invention contains two bypass circuits, one for compression,9, and one for expansion, 10. It should be obvious that an unrestrictedcircuit 9 would allow the flow under compression to bypass thethrottling valve 5, so that the piston 6 would meet a very low degree ofdamping. The same is true about circuit 10 in expansion, which wouldbypass the throttling valves 4.

Now the bypass circuits 9 and 10 are not unconditionally open andunrestricted for the flow. The circuit 9 is provided with a valve C1 andthe circuit 10 with a valve C2. Fundamentally these two valves of reliefhave only two modes, namely free, i.e. unlocked, or locked, i.e. in aclosed position.

There are of course several valve designs. Thus one arrangement containsrelay operated valves, which may be of several alternative designs like"direct relay on valve" or "small valve directly operating a largerslave valve opened by control of pressure by means of the small valve".

However, a very effective and low power valve is now introduced. Herevalves C1 and C2 are springloaded check valves, which are closed whenthe flow is in the directions indicated by arrows D1 and D2respectively.

When the flow is in the directions indicated by arrows E1 or E2,respectively, there could be a flow across the check valves. Thoserespective flows are, however, conditioned upon the action of relays R1and R2. These electromagnetic relays R1 and R2 do not act directly onthe valves C1 or C2. Instead they control a locking device, which eitherfix the valves in a closed position or leave the valves free to openwhen subjected to a pressure difference by a flow in the E1 or E2direction.

The construction of the locking device may follow different designprinciples. One could be a pair of jaws as in a tool (pincers, pliers)with a very small and effective motion. Another tested design isindicated in FIG. 1 and consists of a small springloaded lever 11 or 12,which may be placed in any of two positions 11-11', 12-12' by therespective relay R1 or R2.

This locking design requires a very small amount of power since therelay does not work against the fluid pressure. The motion need not belarge and the locking mechanism has very small inertial forces. Switchtimes of the order of a few milliseconds are easily achieved with thiskind of locking devices.

CONTROL MEANS

The control means for the shock absorber of the active vehiclesuspension unit consist of one or two sensors 22, an electronicmicroprocessor unit 21 (CPU) and a power circuit to activate the relaysR1 and R2.

The basic sensor needed to control the shock absorber for one wheel in agiven vehicle is an accelerometer. This sensor may be attached to thecar body immediately above the wheel, Embodiment I, or it may be fixedto the wheel support, i.e. to the unsprung mass, Embodiment II.

In Embodiment I a second sensor is used to count the number of cycles ofthe shock absorber (from compression to expansion and back tocompression) in a given short time interval. This second sensor could beof different types. Thus a simple pressure sensor in circuit 9 or alimit switch device, preferably contactless, on C1 would suffice.

The relay switch time is easily decreased considerably by using acapacitor charge power circuit which gives the relay a momentary currentmuch larger than the relay would withstand in steady state.

The microprocessor unit 21 comprises an A/D circuit to convert theaccelerometer analogue signal into a digital signal to be processed bythe CPU. Further the CPU unit has a ROM memory containing one or moreprograms according to the selected control principle of different modesof operating the active vehicle suspension unit. Therefore theelectronic unit primarily comprises several input ports for thesensor(s), and A/D circuit, a CPU, a permanent memory ROM and two outputports to activate the power circuits of the relays R1 and R2.

CONTROL PRINCIPLES

When the vehicle is running along a smooth, flat road one desires thesuspension system to operate in a fundamental mode, the zero mode orMODE 0. Depending upon where the sensor/accelerometer is placed thefundamental mode will be defined differently. The desirable generalbehaviour of the shock absorber requires that the fundamental Mode 0will be abandoned temporarily when the wheel is negotiating an upward ora downward edge, see FIG. 2 and FIG. 3 respectively.

Thus, consider what happens when a wheel passes an upward and a downwardedge respectively.

When the wheel hits the upward edge of FIG. 2 the shock absorber will berapidly compressed and the vehicle will be pressed upwards. It isobviously desirable that the shock absorber will absorb the obstaclewith a minimum of resisting force, i.e. the damping should be low orsoft. Passing an upward edge y is positive and (y-x) is negative andtherefore S=y(y-x) is a negative quantity. In the final phase when thewheel has passed the upward edge the vehicle body will still be movingupwards (y>0) and "overshoot" the final equilibrium level when thespring recoils and the shock absorber is expanding, (y-x)>0. One wouldlike to prevent this overshoot by exerting a stiff damping force. Sincethe compression has changed to expansion the sign of S has also changedand become positive.

When passing a downward edge as indicated in FIG. 3 the wheel will tendto loose contact with the road surface. The shock absorber is expandingand the vehicle body tends to fall. At this instant we like theexpansion to be rapid to restore the contact quickly. The sign of S isnegative since y is negative and (y-x) is positive.

Again we see that the desired soft damping is indicated by the negativesign of S. Finally in the ultimate phase of the passage of the downwardedge there will be a tendency to "overshoot" so that the car has amovement to "bottom" into the shock absorber. One would like the dampingto be stiff and the downward motion y<0 in combination with thecompression, (y-x)<0, creates a product of two negative quantities,therefore S is positive, once again indicating that we should put on astiff damping.

The conclusion is that the desirable behaviour of the suspension systemcould be expressed in the following table:

                  TABLE I                                                         ______________________________________                                        Required damping at edges.                                                                Before edge After edge                                            ______________________________________                                        Upward edge   .y > 0        .y > 0                                                          (upwards)     (upwards)                                         (FIG. 2)      (.y - .x) < 0 (.y - .x) > 0                                                   (compression) (expansion)                                       Mode 1        SOFT damping  STIFF damping                                     Downward edge .y < 0        .y < 0                                                          (downwards)   (downwards)                                       (FIG. 3)      (.y - .x) > 0 (.y - .x) < 0                                                   (expansion)   (compression)                                     Mode 2        SOFT damping  STIFF damping                                     ______________________________________                                    

Summarizing the above we may immediately translate the results regardingthe Mode 0 and the behaviour at an upward edge, Mode 1, and at downwardedge, Mode 2, into a corresponding control scheme for the C1 and C2valves:

                  TABLE II                                                        ______________________________________                                        Valve control scheme                                                                    Vertical   Valve    Valve                                           Mode      velocity .y                                                                              C1       C2     Damping                                  ______________________________________                                        Mode 0                                                                        Embodiment I                                                                            0          free     free   SOFT                                     accelerometer                                                                 for  ..y on car                                                               Mode 0                                                                        Embodiment II                                                                           0          locked   locked STIFF                                    accelerometer                                                                 for  ..x on wheel                                                             support                                                                       Mode 1                                                                        (upward   .y > 0     free     locked SOFT                                     edge)     (positive)                 at com-                                                                       pression                                                                      STIFF                                                                         at expan-                                                                     sion                                     Mode 2                                                                        (downward .y < 0     locked   free   STIFF                                    edge)     (negative)                 at com-                                                                       pression                                                                      SOFT                                                                          at expan-                                                                     sion                                     ______________________________________                                    

Valve control given in table II requires that one has integrated they-signal over time to obtain y.

However, it has been found that one could use the acceleration signal ydirectly in the electronic data processing for the control of thevalves, thus making integration unnecessary. The acceleration signal yis obviously an earlier indication of an approaching edge than is theintegrated signal y. On the other hand, the acceleration signal is moresensitive and more noisy. This particularly is the case when the sensoris an accelerometer (for x) fixed to the unsprung mass as in EmbodimentII.

This fact has also been reflected in the design of Mode 0, whereEmbodiment II has a stiff damping whereas Embodiment I has a softdamping. The detection of an upward edge for Embodiment I is of coursesomewhat slower than for Embodiment II, because in the first case thesensor is on the car body. However, compare the sequence of events forthe two embodiments:

EMBODIMENT I Transfer from Mode 0 to Mode 1, see Table II

Since C1 is already in the free mode, the first part of the transfer isalready done. The second part should have C2 transferred to the lockedstate, but that can easily be done without complication during the firstpart of the encounter with the upward edge.

Transfer from Mode 0 to Mode 2, see Table II

The first part calls for a free C2, which is already the case. Againthere is time to prepare the locking of the C1.

EMBODIMENT II

In Embodiment II, C1 and C1 are locked. Therefore there is no immediatepreparedness to meet a roughness in the road. Thus the shift of valvemode must occur in the beginning of the first part of the encounter withthe edge. However, the sensor is on the wheel and the edge is detectedearlier. Since the control in the practical case is made directly fromthe acceleration signal the edge will be detected quite early andtherefore the result for Embodiment II will be similar to that ofEmbodiment I.

Control to Transfer Between the Different Modes 0, 1 and 2

To suppress small, noisy signals from the road and from the different,small scale vibrations in the car body it is necessary to introduce atolerance band ±a m/s² of acceleration. Within the tolerance band ±a theMode 0 is to prevail.

Outside the band there will be a transfer to Mode 1 or Mode 2. Once atransfer to either one has occurred the new mode will be retained a holdtime t_(h) even after the signal magnitude has returned to stay withinthe tolerance band. Therefore one has the following scheme in common toboth Embodiments:

                  TABLE III                                                       ______________________________________                                        Mode changes at accelerations exceeding ±a m/s.sup.2                       Accelerometer                                                                 signal       Mode       Remark                                                ______________________________________                                        -a <  ..y or Mode 0 a   No change of Mode                                      ..y or ..x > a                                                                            Mode 1     The Modes 1 or 2 resp.                                 ..y or  ..x < -a                                                                          Mode 2     will be retained t.sub.h sec                                                  after the signal returns                                                      inside the tolerance                                                          band.                                                 ______________________________________                                    

Optimation of Suspension Characteristics

All conventional suspension systems are compromises between the demandfor ride comfort and the need for good contact between the wheel and theroad (road holding).

The ride comfort is disturbed by the car heaving in a frequency rangecorresponding to the natural oscillation of the car body on itssuspension springs. The second natural vibration occurs when theunsprung wheel system flutters on a wash board road with relativelysmall displacements of the car body. This results in a bad road contactfor the wheel. Depending on the amplitude of the road roughness theshocks imparted by one or several wheels to the car body may be loweringthe ride comfort.

It is the merit of the present invention that it allows a much moresophisticated approach to the optimation of a vehicle suspension inharmony also with the braking and steering actions and correspondingconditions.

The acceleration sensor of Embodiment II or the pulse counter ofEmbodiment I would either one enable the electronic unit to recognizethe occurrence of wheel flutter.

This flutter would occur typically at a natural frequency five timesthat of the car on its springs. The suppression of the wheel flutter isof course desirable in order to improve the road holding of the car. Wemay thus introduce still another Mode 3, with both check valves C1, C2in a locked position. Mode 3 is equal to Mode 0 for Embodiment II, butit is convenient to consider Mode 3 as a separate mode depending on thenumber n_(c) of pulses counted in a given short time interval, "COUNT",which is tuned individually for any given vehicle model.

The microprocessor unit would allow a large number of differentcombinations to connect road holding with ride comfort, braking orsteering actions. Therefore the electronic unit may have several extrainput ports relating the suspension to the braking or steering systems.The following Table IV demonstrates the basic scheme for the transferfrom Mode 0, 1 or 2 to Mode 3:

                  TABLE IV                                                        ______________________________________                                        Modes related to COUNT.                                                       Counter below                                                                            Counter above                                                      critical n.sub.c                                                                         critical n.sub.c                                                                           Remark                                                ______________________________________                                        No action  Transfer to  The transfer to Mode 3                                           Mode 3, both may be optional.                                                 C1 and C2    Thus the driver may                                              locked       overrule the transfer                                                         by a manual command.                                  ______________________________________                                    

I claim:
 1. Vehicle suspension system with one or more shock absorbers,each shock absorber comprising,a hydraulic cylinder, receiving thereinhydraulic liquid, a piston working in said cylinder, partitioning theinterior of said cylinder into an upper chamber and a lower chamber,damping force generating means controlling flow from said upper andlower chambers including adjustable valve means controlled by controlmeans to be in a soft mode to provide soft damping characteristics or ina stiff mode to provide stiff damping characteristics in either thecompression stroke or the expansion stroke of said shock absorber, saidcontrol means being responsive to signals from sensors for signaling thevertical speed (y, x) and/or acceleration (x, y) of the vehicle body (y)or the wheel (x), said signal being entered into a signal processor insaid control means to impart either of said soft or stiffcharacteristics by means of said adjustable valve means, comprisingfirstvalve means providing a variable damping force in the compressionstroke, and second valve means providing a variable damping force in theexpansion stroke, characterized in that said first valve means include afirst controllable valve which is closed to flow during said expansionstroke and which has two states of operation during said compressionstroke under the control of said control means, namely on one hand aclosed state in which the flow path from said lower chamber is closed toprovide a relatively stiff damping mode by the further valve means and,on the other hand an open state in which the flow path from said lowerchamber is relatively unrestricted to provide a relatively soft dampingmode, and said second valve means include a second controllable valvemeans which is closed to flow during said compression stroke and whichhas two states of operation during said expansion stroke under thecontrol of said control means, namely on one hand a closed state inwhich the flow path from said upper chamber is closed to provide arelatively stiff damping mode by the further valve means and, on theother hand an open state in which the flow path from said upper chamberis relatively unrestricted to provide a relatively soft damping mode. 2.A vehicle suspension system according to claim 1 which is furthercharacterized by said control means being programmed, such that whenbody acceleration is sensed said first and second valve means are in azero mode when the acceleration signal (y) from the vehicle body iswithin predetermined values (-a<y<+a), wherein said zero mode ischaracterized by both said valve means being placed in their soft stateswhen the vehicle body acceleration signal (y) is not exceedingpredetermined limit values of the acceleration signal (±a).
 3. A vehiclesuspension system according to claim 1, characterized by said controlmeans being programmed such that when wheel acceleration (x) is sensed,said first and second valve means are in a zero mode when theacceleration signal (x) from the vehicle wheel is within predeterminedvalues (-a<x<+a), wherein zero mode is characterized by both said valvemeans being placed in said firm states when the vehicle wheelacceleration signal (x) is not exceeding predetermined limit values ofthe acceleration signal (±a).
 4. A vehicle suspension system accordingto claims 2 or 3, characterized by said control means being programmedto put said first and second valve means in a first mode when either ofthe acceleration signals (y, x) exceeds the high limit value (+a), toimpart said soft state to the first valve means and said firm state tosaid second valve means to thereby effect a soft compressioncharacteristic and a stiff expansion characteristic to said shockabsorber.
 5. A vehicle suspension system according to claim 2 or 3,characterized by the control means being programmed to put said firstand second controllable valve means in a second mode when either of theacceleration signals (y, x) passes below the low limit values (-a), toimpart said firm state to said first valve means and a soft state tosaid second valve means to thereby effect a stiff compressioncharacteristic and a soft expansion characteristic to said shockabsorber.
 6. A vehicle suspension system according to claim 4,characterized by the control means being programmed to retain said firstmode or second mode for a predetermined time (t_(h)) after theacceleration value has reentered within the acceleration tolerance banddefined by the limit values (-a and +a) and after the expiration of thepredetermined time (t_(h)) to return to said zero mode.
 7. A vehiclesuspension system according to claim 5, characterized by the controlmeans being programmed to retain said first mode or second mode for apredetermined time (t_(h)) after the acceleration value has reenteredwithin the acceleration tolerance band defined by the limit values (-aand +a) and after the expiration of the predetermined time (t_(h)) toreturn to said zero mode.
 8. A vehicle shock absorber according to claim1, characterized by the control means being programmed to put said firstand second valve means in a third mode when a signal to said controlmeans representing the counting of the number of cycles within a giventime interval exceeds a predetermined value (N_(c)), or the accelerationsignal time derivative (x or y) exceeds a given valve (b m/s³) saidcontroller imparting to both of said value means said firm state. 9.Vehicle suspension system with one or more shock absorbers, each shockabsorber including a generally vertical cylinder receiving thereinhydraulic liquid, a piston working in the cylinder and partitioning theinterior thereof into two liquid chambers,an upper and lower chamber,two sets of one or more damping force generating valve means mounted onthe piston for regulating flow between the lower and the upper chambers,a piston rod secured to the piston extending through the upper end ofthe cylinder to the outside, said force generating valve meansconstituting connections between the two liquid chambers, a liquidreservoir connected to the lower liquid chamber by a third set of checkvalves generating a damping force in the compression stroke and allowinga return free flow from the reservoir to the lower chamber in theexpansion stroke, characterized by a circuit forming a connectionbetween the two liquid chambers, a further circuit connecting thereservoir to the lower liquid chamber, each of the two circuitsincluding a controllable first check valve and second check valve,respectively, that can each take two states of operation under theaction of a control means, in a first state locking the valve in aclosed position cutting off the forward flow through the check valve,and in a second state allowing a free forward flow through the checkvalve, whereas the check valve in the first state as well as in thesecond state block the reverse flow of both circuits.
 10. Vehiclesuspension system according to claim 9, characterized by the controlmeans comprising a locking device keeping the check valve head insealing engagement with the valve seat to cut off the forward flow. 11.Vehicle suspension system according to claim 10, characterized by thelocking device comprising a rocking lever barring the valve head in itssealed engagement in one position, and letting the valve head free inone or more further positions.
 12. Vehicle suspension system accordingto claim 9, characterized by that the control means for the check valvescomprises electric relays.
 13. Vehicle suspension system according toclaim 2, characterized by the control means being under the action ofsignals from sensors for stating the vertical speed and/or accelerationof the vehicle body or the wheel (y, x, y, x), said signals beingentered into a CPU micro processor located within said control means.14. Vehicle suspension system according to any of claims 9-13,characterized by the control means being programmed to put the checkvalves in a basic mode or zero mode,such that when the accelerationsignal from the vehicle body or the wheel (y and x, respectively) iswithin predetermined values (-a<x or y<+a), to impart a free state orsecond state, allowing free forward flow to both check valves in casethe vehicle body acceleration signal (y) is not exceeding apredetermined limit value of the acceleration signal (±a m/s²), and toimpart a locked state or first state, to both check valves in case thewheel acceleration signal (x) is not exceeding a predetermined limitvalue of the acceleration signal (±a m/s²).
 15. Vehicle suspensionsystem according to any of claims 9-13, characterized by the controlmeans being programmed to put the check valves in a first mode,wheneither of the acceleration signals (y, x) exceeds the high limit value(±a m/s²), to impart a free, second state, to the first check valve anda locked, first state, to the second check valve, meaning a softcompression and a stiff expansion of the shock absorber.
 16. Vehiclesuspension system according to any of claims 9-13, characterized by thecontrol means being programmed to put the check valves in a secondmode,when either of the acceleration signals (y, x) pass below the lowlimit value (-a m/s²), to impart a locked, first state, to the firstcheck valve and a free, second state, to the second check valve, meaninga stiff compression and a soft expansion of the shock absorber. 17.Vehicle suspension system according to claim 13, characterized by thecontrol means being programmed to retain the first mode or the secondmode for a predetermined time (t_(h) sec) after the acceleration valuehas reentered within the acceleration tolerance band, defined by thelimit values (-a m/s² and +a m/s²) and after the expiration of thepredetermined time (t_(h) sec) to return to zero mode.
 18. Vehiclesuspension system according to claim 16, characterized by the controlmeans being programmed to retain the first mode or the second mode for apredetermined time (t_(h) sec) after the acceleration value hasre-entered within the acceleration tolerance band, defined by the limitvalues (-a m/s² and +a m/s²) and after the expiration of thepredetermined time (t_(h) sec) to return to zero mode.
 19. Vehiclesuspension system according to any of claims 9-13, characterized by thecontrol means being programmed to put the check valves in a thirdmode,when a signal representing the counting of the number of cycleswithin a given time interval exceeds a predetermined value (N_(c)), orthe acceleration signal time derivative (x or y) exceeds a given value(b m/s³), said third mode imparting to both check valves a locked, firststate.